The present invention relates in general to a shielded enclosure for a printed circuit board, and more specifically to an enclosure providing a plurality of compartments shielded from the external environment and from each other wherein the enclosure is easily and inexpensively assembled using sheet metal components.
The radio frequency (RF) shielding characteristics of modules containing electronic circuits is a vital concern in electronic design. Circuits which are susceptible to electromagnetic interference from other sources are shielded to insure proper operation. Circuits generating unintentional RF radiation are shielded to contain and reduce electromagnetic interference (EMI) that may be transmitted to other devices.
Circuits using digital components have a particular need for shielding. The relatively high frequency digital pulses used in digital components comprise square waves that generate broadband electromagnetic interference. Although digital components by their nature have lower susceptibility to interference from other sources, typical electronic circuits employ combinations of digital and analog components. For example, both radio receivers and cellular telephone transceivers use analog RF components for handling radio signals (e.g., mixing radio signals) and digital components for controlling circuit operation or digitally processing audio signals.
In order to alleviate interference between digital and analog components in a single electronic circuit, separate printed circuit boards or multi-layer circuit boards have been used to physically separate or shield digital and analog components. However, such measures increase the cost of a circuit module. It is desirable to employ a single circuit board of minimal size in order to reduce cost of the circuit board itself and the associate module.
Besides separating analog from digital components, shielded compartments are also useful for preventing interference between high level output sections and low level input sections in many types of electronic devices, such as audio eqiupment.
It is also known to employ a circuit housing having sub-chambers to isolate components on a printed circuit board, as shown in Tam, U.S. Pat. No. 5,107,404. Tam discloses a cast or molded frame and cover plates. Cast screw holes in the frame and self-tapping screws mount a printed circuit board to the frame. Interior walls create sub-chambers for receiving particular components such as an antenna connector. The interior walls are grounded by physical contact with ground traces on the printed circuit board. A set of screws extends through the cover plates and frame to sandwich the entire housing together.
The casting used in Tam is expensive and is undesirably heavy for many applications, such as automotive applications. The surface-to-surface contact between interior walls and ground traces provides a less than optimal ground structure in Tam. Also, the assembly method including the use of many assembly screws is very labor intensive. Thus, a need exists for a shielded circuit module which provides external RF shielding and internal shielding between compartments which is inexpensive, light, easy to assemble, and capable of opening for servicing of interior components.